( 628 ) 



taking at the same time the value ui" /' D at the distance r-\-s from 

 the axis. 



The ale:ebraic sum of the two moments will therefore be 



d 

 07 





and tVn' this we niav wi-iie 



- « r — -, 

 c dt 



(24") 



since, by the equation (VI) 



d ^ 1 ab. 



(rD) = --r--. 

 Or c at 



For the sum of (24') and ("24") we may Avrite 



e d {s f)z) 



r , 



c dt 



whence it is immediately seen that its mean value is for a full 

 period. 



Physics. — Methods and appavatua ti.^ed in the cryogenic laborator?/. 

 III. Baths of vei'y iiniforiu and constant temperature in the 

 cryostat {continueil). A cryosiat of modijied form for appa- 

 ratus of small dimensions. IV. A permanent hath of liquid 

 nitrogen at ordinary and at reduced pressure. V. Arrange- 

 ment of a Bürckhardt-Wp:iss vacuum-pump for use in the 

 circidations for loin temperatures. Communication N". 83 (con- 

 tinued) from the Laboratory at Leiden. By Prof. H. Kamerlingh 

 Onnes. (Read February 28, 1903). 



in. § 6. A cryostat of modijied form for apparatus of small 

 dimensions. If the cross sections of the apparatus that is to be immersed 

 into the bath are small, vacuum glasses may be profitably used in 

 the construction of the cryostat. For, vacuum glasses of comparatively 

 small diameter can then accommodate the stirrer and the temperature 

 indicator in addition to the measuring apparatus. Plate lY shows a 

 cryostat of the kind, viz. the one used in the determinations by 

 Hyndman and myself on the critical state of oxygen. 



Obviously the arrangement could be much simpler, as it was not 

 necessary to watch the liquefied gas streaming from the jet or to use 

 the generated cold vapour for the cooling and as no particles of dust 

 from the leads had to be feared, a filter was not required. (Comp. Comm. 

 51, Sept. '99 § 2. Y^ p. 12). The principles for obtaining a uniform con- 



